In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, one parameter in providing good performance and capacity for a given communications protocol in a communications network is spectrum availability.
The spectrum allocated to some telecommunications standards, such as Long Term evolution (LTE), is limited and may therefore have difficulties to meet future throughput demands. It has therefore been proposed to allow at least some telecommunications standards to operate also in unlicensed frequency spectrum in addition to licensed spectrum. One way to utilize the unlicensed frequency spectrum reliably is to transmit essential control signals and channels on a frequency carrier in the licensed spectrum whereas other control signals, channels, and data is transmitted in the unlicensed frequency spectrum.
The 3rd Generation Partnership Project (3GPP) Release 13 (Rel-13) work item “Licensed-Assisted Access” (LAA) intends to allow devices to also operate in the, today, unlicensed 5 GHz spectrum. The unlicensed 5 GHz spectrum is intended to be used as a complement to the licensed spectrum. Accordingly, devices that connect in the licensed spectrum (with a primary cell, denoted PCell) can use mechanisms such as carrier aggregation to benefit from additional transmission capacity in the unlicensed frequency spectrum (from a secondary cell, denoted SCell). To reduce the changes required for aggregating licensed and unlicensed frequency spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Unlicensed frequency spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, considerations are made regarding coexistence with other systems such as the IEEE 802.11 Wireless Local Area Network (WLAN) standard (known as Wi-Fi). Performing operations of a telecommunications standard in the same manner in unlicensed frequency spectrum as in licensed spectrum could degrade the performance of other systems operating in the unlicensed frequency spectrum since some of these systems are configured such that no transmissions are made once an occupied channel has been detected.
Regulatory requirements may therefore not permit transmissions in the unlicensed frequency spectrum without prior channel sensing. Since the unlicensed frequency spectrum could be shared with other systems intended for similar or dissimilar wireless technologies, it has therefore been proposed to use a so called listen-before-talk (LBT) mechanism. In general terms, LBT involves the device (which intends to use a channel in the unlicensed frequency spectrum for transmission) senses the medium for a pre-defined minimum amount of time and backs off if the channel is busy.
As a non-limiting example, European Regulation EN 301.893, v. 1.7.1 provides the following requirements and minimum behavior for a load-based clear channel assessment (CCA). Such a CCA can be used as part of an LBT mechanism.
Step 1) Before a transmission or a burst of transmissions on an Operating Channel, the device shall perform a Clear Channel Assessment (CCA) check using “energy detect”. The device shall observe the Operating Channel(s) for the duration of the CCA observation time which shall be not less than 20 μs. The CCA observation time used by the device shall be declared by the manufacturer. The Operating Channel shall be considered occupied if the energy level in the channel exceeds the threshold corresponding to the power level given in Step 5 below. If the device finds the channel to be clear, it may transmit immediately (see Step 3 below).
Step 2) If the device finds an Operating Channel occupied, it shall not transmit in that channel. The equipment shall perform an Extended CCA check in which the Operating Channel is observed for the duration of a random factor N multiplied by the CCA observation time. N defines the number of clear idle slots resulting in a total Idle Period that need to be observed before initiation of the transmission. The value of N shall be randomly selected in the range 1 . . . q every time an Extended CCA is required and the value stored in a counter. The value of q is selected by the manufacturer in the range 4 . . . 32. This selected value shall be declared by the manufacturer. The counter is decremented every time a CCA slot is considered to be “unoccupied”. When the counter reaches zero, the device may transmit.
Step 3) The total time that a device makes use of an Operating Channel is the Maximum Channel Occupancy Time which shall be less than ( 13/32)×q ms, with q as defined in Step 2 above, after which the device shall perform the Extended CCA described in Step 2 above.
Step 4) The device, upon correct reception of a packet which was intended for this device, can skip CCA and immediately proceed with the transmission of management and control frames (e.g. acknowledgement (ACK) and Block ACK frames). A consecutive sequence of transmissions by the device, without it performing a new CCA, shall not exceed the Maximum Channel Occupancy Time as defined in step 3 above. For the purpose of multi-cast, the ACK transmissions (associated with the same data packet) of the individual devices are allowed to take place in a sequence.
Step 5) The energy detection threshold for the CCA shall be proportional to the maximum transmit power (PH) of the transmitter: for a 23 dBm equivalent isotropically radiated power (EIRP) transmitter the CCA threshold level (TL) shall be equal or lower than −73 dBm/MHz at the input to the receiver (assuming a 0 dBi receive antenna). For other transmit power levels, the CCA threshold level TL shall be calculated using the formula: TL=−73 dBm/MHz+23−PH (assuming a 0 dBi receive antenna and PH specified in dBm EIRP).
Despite the above presented procedure, there is still a problem of how to handle data transmissions during LBT failure.